That's a very tough situation. In my opinion I think it's best to do what you can to meet her wishes. The real issue is probably her overall condition and along with her age, any surgery may not be advisable, not to mention the chemo and recovery issues that you would have to deal with. At some institutions colon cancer is not treated at 80 years or older. The colonoscopy itself carries little danger to the patient, but again what would you do if you could positive results from a biopsy? And that is just the colon, if you do a biopsy of anything found during the colonoscopy you'd almost forced to have to do a biopsy of the liver as well. If I were the person who at 85 had come to the realization that I might have cancer, and that it might be stage 4, (especially after having cancer at age 70), I would probably seek to spend my last days in the most pleasant way possible with family and friends.